This invention relates to two-phase liquid metal magneto-hydrodynamic (MHD) power generators. More specifically, this invention relates to a method for promoting the formation of a foam and for improving bubble retention and foam lifetimes in liquid metal NaK or sodium used to generate power in two-phase liquid metal MHD generators.
In a two-phase liquid metal MHD generator, a compressed, hot, inert gas is used as the thermodynamic working fluid to electrically drive a conductive liquid metal such as NaK, sodium or tin through the generator channel. The gas and liquid are mixed together just as the mixture enters the generator channel so that the expansion of the gas drives the conductive liquid across the magnetic field, generating electrical power. The two phases are then separated and returned to the mixer through different loops.
One problem which has been found to cause MHD generator inefficiencies is the inhomogeneity of the fluid as it passes through the channel. This has been found to be caused by the movement of bubbles of working fluid away from the wall and the rapid coalescence of the bubbles at the center of the channel to form a fast moving gas phase leaving a slower moving annular flow or slug flow of liquid metal around the perimeter of the channel, increasing the ratio of boundary layer-to-core electrical conductivity, and limiting the achievement of high conversion efficiencies.
Ideally, the two-phase gas and liquid metal mixture should flow through the channel at the same velocity while maintaining a uniform void distribution throughout the liquid metal in order to achieve the power generation efficiencies necessary to make the system effective.
One solution to this problem which has been proposed would utilize the surface active property of dilute liquid metal solutions to permit creation of a foam flow which would then pass through the channel in a more or less homogeneous manner. In non-surface active or pure systems, coalescence of mutually encountering bubbles is virtually instantaneous, yielding a slug flow at void fractions higher than 25%. The addition of a small amount of surface active agent or agents that tend to concentrate at the interface, modifying the surface properties of surface tension and elasticity, and creating dynamic effects, may completely prevent coalescence of bubbles for a certain period of time. This may yield adequately stable, homogeneous foam flows with velocity slip ratios near unity at high void fractions.
Thus the problem is to find some element or compound which when added to two-phase liquid metal MHD fluid will promote the formation of liquid metal foams which are stable for a sufficient period of time to provide improved void fractions and gas dispersion as the foam passes through the MHD generator channel, thereby improving generator performance and efficiency.